Conventionally, there has been a demand for utilization of surplus heat generated from automobiles or thermal energy of sunlight (e.g., electromagnetic wave of longer wavelength than the near-infrared). Generally, there was a problem that a scene where a heat is generated (time) and a scene where the heat is (or is desired to be) utilized do not necessarily coincide with each other in time.
For example, in the case of getting in an automobile during winter season, if exhaust heat such as heat generated during driving could be utilized for warming up the engine when getting in the automobile next time (e.g., the next morning), it would bring many advantages. For example, shorter initial warm-up time enables less fuel consumption at the time of engine start, that is, a fuel consumption rate can be improved. Further, immediate use of air heating at almost the same timing as getting in the automobile is also a merit for increasing the comfortability for automobile occupants.
Taking housing as an example, if thermal energy of sunlight in daytime could be utilized for any time after sunset (e.g., night of the day, night of the next day, etc.), then the costs of lighting and heating could be cut down.
One approach for solving the aforementioned problems to enjoy these advantages is a method for storing energy by means of a heat storage material utilizing a material of which three states can change in accordance with temperature increase.
As such heat storage materials, there were conventionally those using sodium pyrophosphate, an ionic liquid, glacial acetic acid, erythritol or the like (Patent Documents 1 to 4).